When automotive speed records are talked about, engine power and a slippery, aerodynamically shaped, body are the first thoughts. Weight, too, is a thought. Then there is the type of vehicle and it’s fair to think of jet or rocket powered cars. Australia’s Rosco McGlashan, for example, is finalising his Aussie Invader 5 rocket car for a tilt at the outright speed record. But what about location?
Bugatti recently faced that question head on when it came to attempting and creating a supercar speed record of 304.773 miles per hour or 490.484 kilometres per hour. Although the Chiron based car, driven by British born Bugatti chief test driver Andy Wallace, had been lengthened in the body by chassis maker Dallara for a better airflow, had a modified exhaust for the otherwise standard quad-turbo 1,600hp W16 engine, and had been lowered in height, Bugatti had looked into a couple of locations for the attempt. As it turns out, there’s some “Big Bang Theory” style physics involved.
In Germany is a state known as Lower Saxony. It’s situated in the north-west of the country, and it’s home to a very special part of the automotive world. It’s the Ehra-Lessien high speed bowl. There are three lanes and the track is 21 kilometres, or 13 miles, in length. Naturally, safety is crucial, and at the northern and southern ends are high quality safety facilities should things go awry. That is a positive, straight away. However, the physics comes from the height of the location itself. Ehra-Lessien is virtually at sea-level, which means air pressure and density is higher than a location even just 1,000 metres higher. The actual molecules of air are more tightly compressed at sea level and as height increases, that density decreases as a result of the pressure falling off.
Nevada, in the west of the continental United States, plays host to many high speed attempts. Neighbouring Utah has the Bonneville salt flats, and these are 1,291 metres above sea level. This height difference has the benefit of having air pressure at around 86% to 88% lower than at sea level, such as that found at Ehra-Lessien. This effectively means that less engine effort is required to achieve, theoretically, the same speed at sea level.
There is a name for the relationship between inertial forces and frictional forces. This is called the Reynolds Number. This equation is then used with air pressure & density and a vehicle’s drag coefficient. Climb a mountain and the Reynolds Number decreases in correlation with the density of the air. To use 1,000 metres as a yardstick density has decreased by around ten percent, and the Reynolds Number also has decreased by ten percent. However it’s been calculated that the Reynolds Number is still at a level that has a vehicle’s drag coefficient virtually equaling what it would read at sea level. This became part of Bugatti’s choice in location, with the safety facilities becoming the sealer of the deal.
Although it was calculated that the Bugatti Chiron could have seen a v-Max of 329mph, possibly even 330mph, by undertaking the attempt in the U.S., the final differences were, in Bugatti’s opinion, not worth the effort needed to get to the proposed Nevada location. It was deemed that the safety factor in Germany was higher and the chance of a “mere” extra 25mph by using a higher location was outweighed in the safety stakes because of a one way track, meaning if there had been an incident, safety vehicles would take longer to reach the site.
As it is, Bugatti have created a new supercar speed record and they’re now content to leave that area of challenge to focus on further developments of their range.